THE STRUCTURE OF THE ROOT 



35 



elongated cells form a definite number of columns of 

 cells which can be traced separately upwards. They 

 are fitted especially to transport water by changes in 

 the constitution of their cell walls, which become 

 gradually changed from cellulose to lignin, the latter 

 enabling water to pass through it in all directions with 

 great ease. At the same time the horizontal walls of 



st st h TV C' 



FIG. 13. Longitudinal section through a vascular bundle of a stem. 

 5, s', p, p. different types of wood vessel; w, wood fibres; st, sieve 

 tubes; ph, bast fibres; p', pith; c, cambium. 



these cells in great part disappear, so that the columns 

 of cells become changed into hollow tubes, or vessels, 

 while their side walls are irregularly thickened by the 

 deposit of more cell-wall substance upon them in 

 particular areas. On account of the presence of these 

 vessels, the collections are known as vascular strands or 

 vascular bundles (Fig. 13). In the root they are com- 

 posed, entirely of lignified cells and are therefore called 

 wood or xylem bundles, to distinguish them from other 

 vascular strands lying near them. The number of these 



